Medical imaging technologies can provide detailed information useful for differentiating, diagnosing, or monitoring the condition, structure, and/or extent of various types of tissue within a patient's body. In general, medical imaging technologies detect and record manners in which tissues respond in the presence of applied signals and/or injected or ingested substances, and generate visual representations indicative of such responses.
A variety of medical imaging technologies exist, including Computed Tomography (CT), Positron Emission Tomography (PET), Single Photon Emission Computed Tomography (SPECT), and Magnetic Resonance Imaging (MRI). Particular imaging techniques, for example, certain MRI techniques, may scan a volume of tissue within a region of anatomical interest. Scan information or data corresponding to an anatomical volume under consideration may be transformed into or reconstructed as a series of planar images or image “slices.” For example, data generated during a breast MRI scan may be reconstructed as a set of 40 or more individual image slices. Any given image slice comprises an array of volume elements or voxels, where each voxel corresponds to an imaging signal intensity within an incremental volume that may be defined in accordance with x, y, and z axes. The z axis commonly corresponds to a distance increment between image slices, that is, image slice thickness.
Any given medical imaging technology may be particularly well suited for differentiating between specific types of tissues. A contrast agent administered to the patient may selectively enhance or affect the imaging properties of particular tissue types to facilitate improved tissue differentiation. For example, MRI may excel at distinguishing between various types of soft tissue, such as malignant and/or benign breast tumors or lesions that are contrast enhanced relative to healthy breast tissue in the presence of Gadolinium DPTA or another contrast agent.
Medical imaging techniques may generate or obtain imaging data corresponding to a given anatomical region at different times or sequentially through time to facilitate detection of changes within the anatomical region from one scan to another. Temporally varying or dynamic tissue dependent contrast agent uptake properties may facilitate accurate identification of particular tissue types. For example, in breast tissue, healthy or normal tissue generally exhibits different contrast agent uptake behavior over time than tumorous tissue. Moreover, malignant lesions generally exhibit different contrast agent uptake behavior than benign lesions (“Measurement and visualization of physiological parameters in contrast-enhanced breast magnetic resonance imaging,” Paul A. Armitage et al., Medical Imaging Understanding and Analysis, July 2001, University of Birmingham).
In general, at any particular time, the intensity of an imaging signal associated with any particular voxel depends upon the types of tissues within an anatomical region corresponding to the voxel; the presence or absence of a contrast agent in such tissues; and the temporal manners in which such tissues respond following contrast agent administration. In several types of breast MRI situations, normal or healthy tissue exhibits a background signal intensity in the absence of a contrast agent, while abnormal or tumorous tissue exhibits a low or reduced signal intensity relative to the background intensity. Thus, prior to contrast agent administration, abnormal tissue typically appears darker than normal tissue. In the presence of a contrast agent, lesions or certain types of abnormal tissue typically exhibit a time-dependent enhancement of imaging signal intensity relative to the background intensity.
In general, a lesion will exhibit one of three types of time dependent contrast agent uptake behavior. Within imaging data corresponding to a time series of scans, each type of contrast agent uptake behavior manifests as a corresponding type of dynamic imaging signal intensity profile or curve. Each type of dynamic intensity curve probabilistically corresponds to whether the lesion is benign or malignant.
FIG. 1 is a graph 100 that generally illustrates a first, a second, and a third type of dynamic imaging signal intensity or relative enhancement curve 110, 120, 130 that may be obtained for a given region of interest (ROI) (i.e., a set of voxels corresponding to an anatomical region) that encompasses one or more portions of a lesion. In the graph 100, the horizontal axis corresponds to time, while the vertical axis corresponds to imaging signal intensity or the extent to which imaging signal intensity is enhanced relative to an initial or precontrast imaging signal intensity.
Prior to contrast agent administration, a precontrast scan is initiated or performed at a first scan time t0. For any given voxel, the precontrast scan establishes a precontrast imaging signal intensity and/or a reference relative enhancement value that may be represented as a variable S0. Contrast agent administration occurs some time after t0 at a time tc, for example, one minute after t0. Essentially immediately after contrast agent administration, the level of contrast agent within vasculature associated with a lesion increases. Imaging signal intensity or relative imaging signal enhancement associated with the lesion correspondingly increases, typically at a rapid rate during an initial time interval. Depending upon lesion characteristics, tissue dependent contrast agent kinetics may subsequently give rise to in an imaging signal intensity curve that (1) continues to increase or enhance; (2) reaches a peak level of enhancement and then levels off or plateaus in an abrupt or generally abrupt manner; or (3) reaches a peak level of enhancement and subsequently declines to lower or reduced levels of enhancement. Each of the aforementioned curve types is respectively referred to as (1) a continued, steady, or persistent enhancement curve 110; (2) a plateau curve 120; and (3) a washout curve 130.
A first postcontrast scan is performed at a first postcontrast scan time t1, which generally corresponds to a time at or near which a peak in imaging signal intensity and/or relative enhancement S1 would be expected in the context of a plateau or washout curve 120, 130. The first postcontrast scan time t1 may be, for example, one minute after contrast agent administration. For ease of understanding, S1 is shown in FIG. 1 as having an identical value for each curve type 110, 120, 130. More generally, each type of curve 110, 120, 130 may correspond to a unique or distinct S1 value.
A second postcontrast scan is performed at a second postcontrast scan time t2, thereby capturing or acquiring for each voxel another corresponding imaging signal intensity and/or relative enhancement value S2. The interval between t1 and t2 is sufficiently large to improve or maximize a likelihood that imaging signal intensity differences corresponding to times t1 and t2 can facilitate categorization of an imaging signal intensity curve as a persistent enhancement, plateau, or washout curve 110, 120, 130. The time between t1 and t2 may be, for example, approximately 4 minutes.
Dynamic imaging signal intensity or relative enhancement curves are typically numerically characterized in accordance with two parameters, namely, a percent enhancement (PE) value and a signal enhancement ratio (SER). For a given voxel, the PE value is defined as the difference between the first postcontrast imaging signal intensity S1 and the precontrast signal intensity S0, normalized relative to the precontrast signal intensity S0. The SER for any particular voxel may be defined as the difference between the first postcontrast imaging signal intensity S1 and the precontrast signal intensity S0, normalized relative to the difference between the second postcontrast imaging signal intensity S2 and the precontrast signal intensity S0. For persistent enhancement, plateau, and washout curves 110, 120, 130, the SER will have a value that is less than 1.0, equal to 1.0, and greater than 1.0, respectively.
PE values exhibit a correspondence to lesion type. In general, a higher PE value may suggest a higher probability that a lesion is malignant. Some existing systems and/or methods for analyzing dynamic MRI data identify a set of voxels corresponding to a highest PE value or a highest intensity as a malignant lesion. Other existing systems and/or methods may identify (1) a set of voxels corresponding to a PE value above a first reference value as a malignant lesion; (2) a set of voxels corresponding to a PE value below the first reference value and above a second reference value as an indeterminate type of lesion; and (3) a set of voxels corresponding to a PE value below the second reference value as a benign lesion (“Dynamic Breast MR Imaging: Are Signal Intensity Time Course Data Useful for Differential Diagnosis of Enhancing Lesions?”, Christiane Kuhl et al., Radiology, April 1999).
Curve type 110, 120, 130 also exhibits a correspondence to lesion type. In particular, a washout curve 130 is strongly indicative of a malignant lesion or weakly indicative of a nonmalignant or benign lesion. A plateau curve 120 may be somewhat more indicative of a malignant lesion than a benign lesion. Finally, a persistent enhancement 110 curve is strongly indicative of a benign lesion or weakly indicative of a malignant lesion. Certain methods for analyzing dynamic MRI data identify a curve shape of washout 130 or plateau 120 as malignant, and a persistent enhancement 110 curve as benign (ibid).
Unfortunately, systems and/or methods that analyze contrast enhanced medical imaging data in manners described above fail to adequately increase or maximize diagnostic sensitivity, specificity, and/or accuracy.